Man-At-Arms is a Heroic Warriors, who is the Master Of Weapons and combat instructor to the royal family, in addition aiding He-Man by leading the Heroic Warriors into battle. He is known for his being a master of strategy and the defense, who works as the senior officer and second-in-command, under King Randor, of the Eternos Royal Guard.

Bio #1 (Pre-Filmation):

In the early minicomics Man-At-Arms is portrayed somewhat differently. His appearance is more in line with the toy, and as there was no royal family yet there is no mention of them. In the first minicomic there is also no mention of his relationship to Teela, who is instead portrayed as a 'warrior goddess'. Instead, the only background given for the character is that his people were "Masters of all weapons." He seems to be well known in this version of Eternia, but little information beyond this is given.

Bio #2 (Filmation):

As Eternia’s “Master of Weapons” Man-At-Arms is in charge of the kingdom’s defenses. Known to his friends as Duncan, Man-At-Arms is a highly skilled inventor and scientist. He is a loyal servant of King Randor and Queen Marlena. He enjoys a close friendship with Prince Adam, his adoptive daughter Teela and Orko. With his intelligence, incredible mechanical skills and fatherly guidance, Man-At-Arms is one of the most valuable and beloved members of the Heroic Warriors. Man-At-Arms was a student of the intellectual named Rohad. After the death of her husband, the Sorceress chose Man –At-Arms to care for her daughter Teela. To this very day Man-At-Arms continues to hold onto the secrets of Teela’s past. The only parent Teela has ever known, Man-At-Arms has given his adoptive daughter all the love, guidance and warrior training she could ever hope for. Man-At-Arms and Teela are very close and will often go to great lengths for each other. King Randor relies on Man-At-Arms to summon He-Man in the event of an emergency. Man-At-Arms and He-Man share a special bond, as he is the only adult besides the Sorceress in Prince Adam’s life with whom Adam can share his secret and ask for advice. While Duncan enjoys a father/son style relationship with Adam, he and He-Man are more like co-workers fighting side by side against Skeletor. Man-At-Arms, like the Sorceress guides He-Man’s development and reminds Adam that he was given his power to help others, not himself. Man-At-Arms usually bears the brunt of Orko’s fouled up magic tricks. It is only because Man-At-Arms cares for Orko that he disciplines him in turn for his follies.

Bio #3 (200X Series):

The200X cartoon portrays Duncan as a much more serious and darker character, with more of a connection to the Sorceress than was previously shown. As in the Filmation series, Man-At-Arms is Teela's adoptive father, but he is also depicted as an officer with the definite rank of General instead of the undefined, but still high-echelon position he holds in the Filmation cartoon. Likewise, Man-At-Arms has a close mentor/student relationship with Adam in addition to aiding He-Man and leading the Masters . It is also revealed that he is the brother of Fisto. It also revealed Duncan's mentor, Dekker.

Bio #4 (Son Of He-Man):

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Bio #5 (Live-Action Movie):

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Bio #6 (MOTUC):

For two centuries, the Eternian weapons master and combat instructor to the royal family has been called the “Man-At-Arms.” Trained by the renowned tactician Dekker and a veteran of the Great Unrest, Duncan was asked by King Randor to step into this position and fortify his guard with an elite strike force, which he named The Masters of the Universe. In addition to his skills in combat, Duncan is also a great inventor and helped construct an electronic version of the Power Sword for Adam to use until he could unite both halves of the sword of King Grayskull. Man-At-Arms and his adopted daughter Teela often scout the borders of Eternia for signs of lurking evil. Duncan heroically served King Randor throughout their battles with Skeletor and the Snake Men as Eternia's chief war strategist and weapons maker. Exposed to the Serpent's Ring, Duncan was magically transformed by King Hsss into a Snake Man permanently. Under this evil spell, he used his genius to create great and horrific machines of war for the Snake Men, as they battled for control of Eternia. Eventually, Duncan was forced to battle against Clamp Champ, who had since replaced him as Royal Man-At-Arms. In their epic final duel during the Second Ultimate Battleground, Duncan was mortally wounded, but not before momentarily recovering his humanity thanks to a spell cast by his daughter Teela, the new Sorceress of Grayskull.